1. Field of the Invention
This invention relates to a photoelectric conversion system having a photoelectric conversion device and a circuitry for driving the device.
2. Description of the Related Art
As a photoelectric conversion device, a MOSFET photocell and photodiode using a monocrystalline silicon, amorphous silicon, polycrystalline silicon or the like are known. A CCD is well known in the art as a system for transferring charges generated in the silicon bulk by application of light by scanning an electric field. Because of the charge transfer inside the bulk, in this system, a so-called semiconductor thin film deposition technique for forming a photoelectric conversion device on the insulating substrate cannot be used and this system is not suitable for a photo device of large area.
An area type photo-array can be formed by arranging photodiodes or MOS photocells on the insulating substrate in a matrix form. In this case, each of the photodiodes or MOS photocells is connected to a switching element such as a MOS TFT and charges stored on the respective photodiodes or MOS photocells are sequentially read out at preset timings.
A novel photosensor with TFT structure is proposed by the inventor of this application. This invention is disclosed in Japanese Patent Application KOKAI publication No. 3-82171, and the outline thereof is explained with reference to FIGS. 9 and 10.
FIG. 9 is an enlarged cross sectional view of a photosensor 1. The photosensor 1 has a structure having a bottom gate electrode 3, bottom gate insulating film 4, photoelectric conversion semiconductor layer 5 formed of amorphous silicon, source electrode 6, drain electrode 7, top gate insulating film 9 and top gate electrode 10 laminated in this order on a transparent glass substrate 2. The top gate electrode 10 and top gate insulating film 9 are transparent. The source electrode 6 and drain electrode 7 are separated from each other so that light can be illuminated on that portion of the semiconductor layer 5 which lies between the edge portions of the electrodes 6 and 7. Illumination light A is illuminated from the top gate electrode 10 side of the photosensor 1. The operation of the photosensor is explained with reference to FIG. 10.
FIG. 10 shows characteristic curves showing the relation between a drain current I.sub.D and a top gate voltage V.sub.TG applied to the top gate electrode 10 using the presence and absence of the illumination light A as parameters in a condition that a bottom gate voltage V.sub.BG =+20 V is applied to the bottom gate electrode 3 and a drain voltage v.sub.d =+10 V is applied between the source electrode 6 and the drain electrode 7. In FIG. 10, the characteristic curve C.sub.0 indicates a case of no light illumination and the characteristic curve C.sub.L indicates a case of light illumination.
In the case of no light illumination, n-channels are formed in both of the upper and lower surface regions of the semiconductor layer 5 when the top gate voltage V.sub.TG =+40 V. As a result, a drain current I.sub.D of several tens micro A (Ampere) can be obtained. The drain current I.sub.D becomes smaller as the top gate voltage v.sub.TG becomes lower, and it becomes smaller than 10.sup.-14 A when the top gate voltage V.sub.TG becomes approximately equal to -20 V. This is considered to be because the n-channel formed in the lower surface of the semiconductor layer 5 by application of the bottom gate voltage V.sub.BG =+20 V is cancelled by application of the top gate voltage V.sub.TG =-20 V. In the case of light illumination, a drain current I.sub.D which is as large as several tens micro A as in the case of no light illumination flows at the time of application of the top gate voltage V.sub.TG =+40 V as might be expected. Unlike the case of no light illumination, the drain current I.sub.D will not be significantly reduced when the top gate voltage V.sub.TG is lowered and a current of approx. 1 micro A will flow even when the top gate voltage V.sub.TG is lowered to -40 V. Therefore, in the photosensor 1, an excellent characteristic in which the ratio of a current (light current) in the case of light illumination to a current (dark current) in the case of no light illumination is set to a number in seven figures can be obtained.
However, even in the photosensor, only the signal-to-noise ratio can be increased, and like the conventional photodiode and MOS photocell, a switching element for reading out stored charges is required.
This invention is made in the above-described situations and an object of this invention is to provide a photoelectric conversion system which can be designed such that the photosensor itself will have both of the photoelectric conversion function and the readout selection function.